1. Field of the Invention
The invention relates to a method in which a laser is employed for producing acoustic frictional resistances particularly as flat structures having a plurality of extremely narrow holes for passages of approximately cylindrical or conical shape, to be used in electroacoustic transducers.
From the publication "Acoustical Engineering" by H. Olson, Publisher Van Nostrand Co., 1957, page 89, it is known that a cylindrical bore having a radius r and length l has its acoustic impedance given by the formula: ##EQU1## with (air density).rho.=1.25.10-3 gcm.sup.-3 and (air viscosity)=1.86.0.sup.-4 gcm.sup.-1 sec.sup.-1. For a conical bore with a small radius r.sub.1, large radius r.sub.2, and length l, the formula reads: ##EQU2##
These two formulas comprise a real term and an imaginary term which represent, respectively, the acoustic frictional resistance and the acoustic reactance formed by the air mass. While the acoustic frictional resistance is independent, the acoustic reactance depends linearly on the frequency and, in an equivalent electrical circuit, corresponds to an inductivity. The frequency at which the absolute values of the real and imaginary terms are equal to each other, is called transfer frequence fu which is given for a cylindrical bore by ##EQU3## and for a conical bore by ##EQU4##
At all frequencies exceeding fu, the reactant consisting of the acoustic mass is predominant; at frequencies which are lower or substantially lower than fu, the bore represents an impedance acting predominantly as an acoustic friction. Consequently, the efficiency of a bore as an acoustic frictional resistance depends, with a certain frequency range, on the diameter or the radius of the bore.
In Austrian Pat. No. 337,793, it is mentioned that in general, materials such as felt, non-woven fabric, tissues of various kinds, woven textiles, and even finely perforated metal foils are employed as acoustic frictional resistances. In the same disclosure, however, structural slots are also mentioned as means for producing acoustic friction, and the fabrication of an acoustic frictional resistance, preferably from a thermoplastic, in the shape of a flat structure with narrow passages is described, which are obtained by providing in the flat structure on both sides a plurality of depressions which partly extend over each other thereby producing sound passages at the overlapped locations.
It may also be provided, however, as does British Pat. No. 670,868 to combine two or more perforated plates with staggered perforations which cooperate with a very narrow air gap formed between the perforated plates, to damp the resonance peaks of a crystal microphone.
It is known from the technology of plastics that very narrow bores, having diameters of 0.5 mm and even less, can be produced with industrial lasers, primarily CO.sub.2 lasers. See in this connection, number 2, 1975, of the periodical "Laser and Laser Optics".
Electroacoustic transducers, particularly high quality transducers to be used in studios, with hi-fi equipment and in the communication technology, are provided, in accordance with their destination and at certain locations, with acoustic frictional resistances. To mention only an example, the oscillating part of an electroacoustic transducer, namely the diaphragm, must be damped in its motion by means of such acoustic frictional resistances, to smooth particular resonance peaks and obtain a uniform frequency response over the transmission range.
The materials provided by the above mentioned Austrian Pat. No. 337,793 for acoustic frictional resistances, namely filter paper, open-pore foamed plastics, metal grids, and metal tissue, are manufactured also for other purposes, not specially as acoustic resistances. For example, felt is employed for hats, non-woven fabric for coating, foamed plastic material for packing, upholstering, and padding, and metal grids for purely technological purposes. Therefore, these materials have the disadvantage of having widely spread damping and frictional properties, or also, in view of the design of an acoustic frictional resistance in the median frequencies range, i.e. starting from 1,000 Hz, of being strongly affected by the acoustic mass. Either extensive and time consuming adjustments must then be provided, or the acoustic mass, acting as a reactance, must be taken into account in the design of the transducer, which may frequently lead to losses in the acoustic quality of the transducer. Another disadvantage of prior art acoustic resistances is that not all of them can be miniaturized as desired, which is at odds with a miniaturization of transducers.